1. Field of the Invention
The invention relates to an output stage of a data carrier as defined in the opening part of claim 1 and to a data carrier as defined in the opening part of claim 9.
2. Background Art
Such a data carrier for contact-bound communication via a communication contact, as defined in the opening part of claim 9 is known from the document WO96/38814 and takes the form of a transponder of a smart card. The known data carrier is adapted to transfer communication data from or to a write/read station via a contact pad of the smart card. The contact pad of the smart card has six contact faces, two power supply contact faces serving to power the data carrier with an external supply voltage, a reset contact face serving to receive reset information, and a clock contact face serving to receive a clock signal. Two further contact faces are communication contact faces, which are each connected to a communication contact of the data carrier and via which the communication data are transferred to the write/read station. Thus, communication data are received via the first communication contact of the data carrier and communication data are supplied via the second communication contact.
The known data carrier includes power supply means to which the external supply voltage can be applied by the write/read station and by which an internal supply voltage can be supplied for the power supply of the electrically active module of the data carrier. Such power supply means are also known from data carriers for the contactless communication via an antenna stage, where an HF signal can be applied to the power supply means by the antenna stage and an internal supply voltage can be supplied for the power supply of the electrically active module of the data carrier. These power supply means have the property that it is not possible to determine the instantaneous power consumption of the data carrier by analysis of the power (HE signal, external supply voltage) applied to the power supply means.
It is very important that these modules of the data carriers are powered with the internal supply voltage (power) only via the power supply means. On the other hand, it would be possible, by applying an additional external supply voltage (analysis voltage) to one of the communication contacts, to impress an analysis current having a supply current strength and to power all the modules of the data carrier with this current. Moreover, it should be avoided that the modules are energized only partly with an analysis current having only a supply current strength, because in such a case a hacker could also detect secret data processed by the data carrier by analyzing the variation of the power consumption in the data carrier as a function of time, which power consumption depends on the mode of operation of the data carrier.
The known data carrier includes processing means (microprocessor) for processing received or stored communication data. The known data carrier further includes an output stage, which is connected to the processing means and the second communication contact, to supply communication data processed by the processing means to the write/read station.
Such an output stage usually includes a control stage, a first transistor (P-channel field-effect transistor) and a second transistor (N-channel field-effect transistor). Both transistors have drain terminals (first output terminal, second input terminal) connected to the communication contact. The first transistor has a source terminal (first input terminal) connected to the internal supply voltage and the second transistor has a source terminal (second output terminal) connected to a reference potential of the power supply means.
The control stage is connected to gate terminals (control terminals) of the two transistors and alternately drives the transistors into conduction, in accordance with the communication data to be supplied. As a result of this, the communication contact is connected either to the internal supply voltage or to the reference potential, in order to supply a high level or a low level to the write/read station as communication data.
It has proved to be a drawback of the output stage of the known data carrier that the first transistor (P-channel field-effect transistor) has a parasitic diode which is poled in the forward direction from the drain terminal to the bulk terminal, which is connected to the source terminal, and which under certain conditions makes enables an analysis current having at least an analysis current strength to be fed in and enables the aforementioned hacking of secret data by a hacker. Moreover, under certain conditions during the supply of a high level of the communication data, i.e. when the first transistor is in its conductive state, an analysis current can be fed in and secret data can be detected by a hacker.
It has proved to be a further drawback of the control stage of the known data carrier that the external supply voltage (analysis voltage) applied to the communication contact may lead to an undesired increase of the internal supply voltage, as a result of which individual modules of the data carrier may be destroyed.
It is an object of the invention to provide an output stage for a data carrier, in which the feed-in of the analysis current having at least the analysis current strength is inhibited so as to guarantee the integrity of the secret data stored in the data carrier and processed by means of the data carrier and so as to prevent modules of the data carrier from being destroyed by an excessive internal supply voltage. This object is achieved by means of the measures defined in the characterizing part of claim 1 for a data carrier output stage as defined in the opening part of claim 1 and by the measures defined in the characterizing part of claim 9 for a data carrier as defined in claim 9.
Thus, it is achieved that even when an external supply voltage (analysis voltage) is applied to the communication contact of the data carrier no analysis current having the analysis current strength or even the supply current strength can be injected into the data carrier via the communication contact. This has the advantage that, on the one hand, a hacker cannot detect secret data stored or processed in the data carrier by injecting the analysis current via the communication contact and the advantage that, on the other hand, modules of the data carrier cannot be destroyed by an internal supply voltage increased by the analysis current.
The measures defined in claim 2 provide an output stage having inhibit means in accordance with the invention, which proves to be particularly advantageous in practice.
The measures defined in claim 4have the advantage that an inhibit means can be realized in a particularly cheap manner as a linear resistor.
The measures defined in claim 5have the advantage that as a result of the limitation of an external supply voltage (analysis voltage) applied to the communication contact the current strength of the analysis current impressed via the resistor and the parasitic diode of the first transistor is kept smaller than the analysis current strength.
The measures defined in claim 6have the advantage that feeding in of the analysis current by applying the external supply voltage (analysis voltage) to the communication contact is completely inhibited.
The measures defined in claim 7 have the advantage that a Schottky diode has a particularly small voltage drop (threshold voltage approximately 0.3 V) in the forward direction. The provision of the Schottky diode has the advantage that it prevents an analysis current and reduces the voltage value representative of the high level of the communication data only to a small extent.
The measures defined in claim 8 have the advantage that when no communication data are supplied from the output stage to the communication contact the first transistor is cut off and the internal supply voltage is not applied to the communication contact as a high level. Consequently, slight variations of the internal supply voltage cannot be analyzed by a hacker in order to detect secret communication data.
The measures defined in claim 10 have the advantage that the N-channel field-effect transistor in the output stage does not have a parasitic diode poled in the forward direction from the drain terminal to the source terminal (bulk terminal), which would enable the injection of an analysis current.
The measures defined in claim 11 have the advantage the inhibit means are formed by the first transistor and the control stage themselves, the first transistor being turned off and inhibiting the application of the analysis current when an analysis voltage in excess of the internal supply voltage is applied to the communication contact.
The measures defined in claim 12 have the advantage that the data carrier with the output stage can be manufactured particularly cheaply.